This invention discloses a novel high efficiency nitrogen rejection unit by which varying amounts of excess nitrogen are removed from a natural gas stream. Transporting pipelines usually accept natural gas containing up to a maximum of four mole percent total inerts. In this disclosure, total inerts are calculated as the sum of carbon dioxide, nitrogen, helium and other non-hydrocarbon gases. Carbon dioxide is easily removed by various commercial methods, for example as taught in my co-pending patent application Ser. No. 07/932,867 filed Aug. 20, 1992 and my U.S. Pat. No. 5,141,544 issued Aug. 25, 1992, and by U.S. Pat. No. 4,762,543.
However, nitrogen, helium and argon are not as chemically reactive and, therefore, cannot be removed as easily or generally by the same methods as carbon dioxide. Nitrogen, helium, argon and other atomically light gases physically act in similar manners at very low temperatures, therefore it will be understood that reference only to nitrogen in the remainder of this description also includes these other gases.
Prior to the above intellectual property matters, commercial removal of nitrogen usually was accomplished by fractionation under cryogenic conditions, as seen, for example in U.S. Pat. Nos. 4,451,275, 4,526,595, 4,675,035, and 4,609,390. These previous nitrogen extraction methods achieve a high degree of nitrogen purity, but at a high cost in initial plant equipment and refrigeration horsepower. Examples of these and other processes are shown in the accompanying Prior Art Statement.
The nitrogen removal method and apparatus presented herein uses no external refrigeration equipment and is considerably less expensive than previously known conventional methods. The process of this invention utilizes a thermal drive mechanism comprising a series of Joule-Thomson expansion valves (sometimes hereinafter referred to as a JT valve), the optimum physical placement of cross heat exchangers, and computer-based automatic control of cross heat exchanger loading and temperature monitoring.
This invention differs from my above mentioned patents and patent application by the provision of method and apparatus that includes a modified thermal drive mechanism which utilizes a series of Joule-Thomson expansion valves and the optimum physical placement of cross heat exchangers, and computer-based automatic control of cross heat exchanger loading and temperature monitoring.